Field of the Invention
The present invention concerns a method in order to acquire MR data (for example to create MR images) as well as a method in order to determine a B1 magnetic field. Moreover, the present invention concerns a magnetic resonance system designed in order to implement one or both of such methods.
Description of the Prior Art
The precise knowledge of the B1 magnetic fields present in the examined region of the examination subject is of great importance for many applications in the field of magnetic resonance tomography, for example for the calculation of an RF pulse in multichannel transmission operation or for quantitative T1 examinations. In a high, static B0 magnetic field (for example of a strength of 3 Tesla or more), distinctive spatially dependent variations of the B1 magnetic field can occur due to subject-specific variations of the conductivity and susceptibility. Therefore, a subject-specific determination of the actual B1 magnetic field that is present is indispensable for many applications given a defined, set transmission power.
One possibility to determine the actual B1 magnetic field is offered by what is known as the Bloch-Siegert method, as described in DE 102010017315A1 and in “B1 mapping by Bloch-Siegert shift”, L. Sacolick et al., Magn. Reson. Med. 2010; 63: Pages 1315-1322. A phase shift of the nuclear spins is thereby measured, which is generated by a non-resonant RF pulse. The B1 amplitude of the B1 magnetic field generated by the non-resonant RF pulse can be calculated using the magnitude of the phase shift.
An accelerated method to determine the actual B1 magnetic field is described in “Fast B1 Mapping using a STEAM-based Bloch-Siegert Preparation”, K. Nehrke et al., Proc. Intl. Soc. Mag. Reson. Med. 19 (2011); Page 4411.
In other known MR methods with which MR images are generated, for example, a stimulated echo is generated which is detected upon readout of the MR data. The problem can occur that a magnetization prepared in a previous sequence negatively affects or adulterates the MR data acquired for the current sequence. For example, artifacts can occur in an MR image reconstructed from the MR data. If such an MR method based on the stimulated echo is used to determine the B1 magnetic field, a phase shift, and therefore the B1 magnetic field amplitude, are disadvantageously determined incorrectly due to this problem.